Low-temperature anomalies in the high-field specific heat of UCd11.

Abstract

VOLUME 44, NUMBER 18 PHYSICAL REVIEW B Low-temperature NOVEMBER 1991-II anomalies in the high-field specific heat of UCd» B. Andraka Department of Physics, University of Florida, Gainesville, Florida 32611 G. R. Stewart Department and University of Physics, University of Florida, of Augsburg, Memminger Strasse Gainesville, Florida 32611 D 8900-Augsburg, Germany Z. Fisk Los A/amos Rational Laboratory, Los Alamos, Xew Mexico 87545 (Received 10 May 1991; revised manuscript received 8 July 1991) The specific heat of UCd» has been investigated in magnetic fields up to 16 T and in the temperature range 0. 35 —10 K. In addition to the well-known 5-K magnetic transition, another anomaly is observed below 2 K in fields larger than 10 T. This anomaly can be related to the previously reported signature in resistivity under pressure. Furthermore, the magnetic fields do not affect the the temperature-dependent high-temperature specific heat (above the 5-K transition). The latter observation is discussed in relation to the origin of the extremely large electronic effective mass of UCd». INTRODUCTION The origin of extremely large electronic effective masses (m *) in materials commonly described as heavy- fermion systems continues to be a subject to intense inves- tigations. ' Although a few possible scenarios for the mass enhancement have been postulated, there is no gen- eral consensus as yet. ' In fact, the possibility that not one, but several different mechanisms lead to the ob- served large m* in this group of materials has recently received some experimental support. UCd» is one of the most interesting heavy-fermion compounds for a number of reasons. It undergoes a phase transittion at temperature close to 5 K. The exact nature of the tran- sition is not well understood, although the magnetic-field dependence of the critical temperature (T~) and results are consistent of the muon-spin-rotation experiment with an antiferromagnetic ordering. However, the shape of the specific heat divided by the temperature versus see Fig. 1) below and temperature squared (C/T vs above the transition is rather uncommon for antiferro- magnetic ordering. A broad tail, from 5 K (T&) to about 8 K, is observed on the high-temperature side of the tran- sition, while a well-pronounced shoulder appears below T~ (at 3 —4 K). Several explanations have been put for- ward to explain this unusual temperature dependence of the specific heat below T~. Temperature derivatives of electrical resistivities obtained under sufficiently high hydrostatic pressure are indicative of two additional However, there has not been found phase transitions. any direct evidence for these transitions from either or specific-heat measurements. magnetic-susceptibility The question whether the anomalies in the resistivities under pressure are related in any way to the unusual specific-heat behavior at ambient pressure remains unanswered. T; The temperature dependence of the specific heat of UCd» well above T~, i.e. , above 8 K, is atypical of the known heavy-fermion systems [with the exception of UzZni7 (Ref. 10)]. C/T is proportional to T up to at least 13 K with an extremely high value of the intercept of the ordinate axis, of about 840 mJ/(K mol). The value of the linear term above 8 K in UCd» is the highest systems (in this temperature among all heavy-fermion range). We have used large magnetic fields up to 16 T in order to suppress the 5-K transition and to find out whether this simple C/T=y+PT relation between the specific heat and temperature persists down to lower tem- peratures or whether a Kondo-like increase in C /T vs T takes place as for many other heavy-fermion systems. RESULTS AND DISCUSSION The specific heat was measured using a time-constant method. The temperature in magnetic fields was deter- mined from a SrTi03 capacitance sensor (above 1 K) and 220-0 Speer resistor (below 1 K), calibrated in zero field against a Ge resistance thermometer. Experiments were performed on two assemblies of single crystals, each of a total weight of about 3 mg of UCd», which has a cubic BaMg» crystal structure. Magnetic fields were applied in the (100) crystallographic direction. data have several interesting The magnetic-field features, including an appearance of a second transition at lower temperatures for sufficiently high fields. Follow- ing the notation of Ref. 8, we denote the upper tempera- ture transition as T, and the lower as T2 and the critical temperatures, as inferred from the maxima in C/T, T, j and T, 2, respectively. Let us discuss first the effects of magnetic fields on T, that can be partially inferred from Fig. 1 and Table I. T„decreases faster than linearly with a magnetic field,